276
resistance to Bromus through effects on soil water storage, timing of water and
nutrient availability, and dominant plant life forms. Differences among plant
communities in how well soil resource use by the plant community matches resource
supply rates can infl uence the magnitude of resource fl uctuations due to either
climate or disturbance and thus the opportunities for invasion. The spatial and tem-
poral patterns of resource availability and acquisition of growth resources by
Bromus versus native species strongly infl uence resistance to invasion. Traits of
Bromus that confer a “priority advantage” for resource use in many communities
include early- season germination and high growth and reproductive rates. Resistance
to Bromus can be overwhelmed by high propagule supply, low innate seed dor-
mancy, and large, if short-lived, seed banks. Biological crusts can inhibit germina-
tion and establishment of invasive annual plants, including several annual Bromus
species, but are effective only in the absence of disturbance. Herbivores can have
negative direct effects on Bromus , but positive indirect effects through decreases in
competitors. Management strategies can be improved through increased under-
standing of community resistance to exotic annual Bromus species.
Keywords Ecosystem resilience • Plant traits • Resource dynamics • Species
interactions • Herbivory
10.1 Introduction
The resistance or conversely susceptibility of plant communities to exotic annual
Bromus species ( Bromus hereafter) and other invasive species is a function of environ-
mental factors, community attributes and processes, life history and ecophysiological
traits of the invader, and direct and indirect interactions of the invader (see reviews in
Chesson 2000 ; Shea and Chesson 2002 ). Resistance to invasion varies both spatially
and temporally (Chesson 2000 ; Seabloom et al. 2003 ) and is closely related to the
resilience of a community (Chambers et al. 2014a ). Resilience is the capacity of a
community to regain its fundamental structure, processes, and functioning when
altered by stressors like drought, and disturbances like inappropriate grazing and
altered fi re regimes (Holling 1973 ; Folke 2006 ); resistance is the capacity of a com-
munity to retain its fundamental structure, processes, and functioning despite stresses,
disturbances, or invasive species (Folke et al. 2004 ). While resilience is a measure of
the recovery potential of a community following stress or disturbance (Chambers
et al. 2014a ), resistance to invasion is a measure of the capacity of a community to
limit the population growth of an invading species (D’Antonio and Thomsen 2004 ).
The relationships of environmental factors , including climate, topography, and soils,
to the abiotic and biotic attributes of communities, and to resilience to disturbance
and resistance to Bromus , are illustrated in Fig. 1.1 (Germino et al. 2015 ).
In this chapter, we examine the interactions of four focal, invasive annual Bromus
species — B. diandrus Roth (ripgut brome), B. hordeaceus L. (soft brome), B. rubens
J.C. Chambers et al.